The present invention relates to a method of and an apparatus for improving the Langelier index and pH of city water for preventing corrosion of city water pipes.
Conventionally, as the technology for preventing corrosion of water pipes, in other words, the technology for preventing red rust water, the caustic soda injection method, the soda ash injection method, the slaked lime injection method and the like are known, however, the slaked lime injection method recently draws attention. The reasons why slaked lime is often adopted in water purifying plants are as follows. (1) The slaked lime not only increases the pH of water, it also improves the Langelier index, (2) it has a calcium component which is said to be also good for health as its main component, and (3) a technique for injecting slaked lime not as powder but as an aqueous solution has been developed (for example, U.S. Pat. No. 4,366,142) and the like.
However, when the slaked lime is injected as an aqueous solution, since a larger or smaller amount of carbonic acid is dissolved in water to be used for producing a slaked lime aqueous solution, it is impossible to avoid the formation of calcium carbonate during dissolution of the slaked lime. In addition, calcium carbonate is formed on the surfaces of undissolved slaked lime particles, so that depending on a carbonic acid content of the water used to dissolve the slaked lime, the amount of solid matters remaining as undissolved matters during production of the slaked lime aqueous solution reaches ten and several % of the raw material slaked lime. Thus, other than a problem of lowering the effective utilization percentage of slaked lime, a problem occurs in the treatment of non-utilized solid matters which is typically discarded.
On the other hand, according to the water quality standard, the pH of city drinking water is determined to be 5.8 to 8.6, however, from a viewpoint of preventing corrosion of water pipes, that is prevention of red rust water, it is preferable that the pH of city water is higher, and it is desired to be at least not less than pH 7.0, and to have the Langelier index of not less than -1.0, preferably not less than -0.5, more preferably -0.5 to 0.5.
Thus, the subject of the present invention is to provide complete utilization of slaked lime, namely to utilize it without remaining any undissolved calcium and to provide a method of improving both the pH and the Langelier index of city water and an apparatus for carrying out it.
In this case, the Langelier index (LI) is an overall index of city water comprising the calcium hardness, pH and alkalinity, which is represented by the following equation: EQU LI=pH-pHs=pH-8.313+log[Ca.sup.2+ ]+log[A]-s
wherein; pH; measured pH value of water to be tested pHs; theoretical pH value of water to be tested when CaCO.sub.3 in the water maintains equilibrium according to the following equation; Ca.sup.++ +HCO.sub.3 .revreaction.CaCO.sub.3 +H.sup.+, log[Ca.sup.2+ ]; logarithm of calcium ion concentration log[A]; logarithm of overall alkalinity s; correction value.